Syntheses and applications of soluble polyisobutylene (PIB)-supported transition metal catalysts

Abstract

Soluble polymer supports facilitate the recovery and recycling of expensive transition metal complexes. Recently, polyisobutylene (PIB) oligomers have been found to be suitable polymer supports for the recovery of a variety of transition metal catalysts using liquid/liquid biphasic separations after a homogeneous reaction. Our work has shown that PIB-supported Ni(II) and Co(II) β-diketonates prepared from commercially available vinyl terminated PIB oligomers possess catalytic activity like that of their low molecular weight analogs in Mukaiyama epoxidation of olefins. Carboxylic acid terminated PIB derivatives can act as carboxylate ligands for Rh(II) cyclopropanation catalysts. An achiral PIB-supported Rh(II) carboxylate catalyst showed good activity in cyclopropanation of styrene in hydrocarbon solvents, and could be easily recycled nine times by a post reaction extraction. Further application of PIB supports in asymmetric cyclopropanation reactions were investigated using PIBsupported arenesulfonyl Rh(II) prolinates derived from L-proline as examples. The PIBsupported chiral Rh carboxylates demonstrated moderate activity and were recovered and reused for four to five cycles. The prolinate catalyst prepared from PIB-anisole also showed encouraging enantioselectivity and about 8% ee and 13% ee were observed on trans- and cis-cyclopropanation product respectively. Finally, PIB oligomers can be modified in a multi step sequence to prepare PIBsupported chiral bisoxazolines that can in turn be used to prepare active, recyclable PIBsupported Cu(I) bisoxazoline complexes for olefin cyclopropanation. These chiral copper catalysts showed moderate catalytic activity and good stereoselectivity in cyclopropanation of styrene. A chiral ligand prepared from D-phenylglycinol provided the most effective stereo control and gave the trans- and cis-cyclopropanation product in 94% ee and 68% ee respectively. All three PIB-supported chiral bisoxazoline-Cu(I) catalysts could be reused five to six times.